5 replies to this topic

[Bambang Wahyu Pramono]

Dear all,

My name is Bambang from Indonesia,
i'm working on my final year project in chemical engineering faculty now,
at first I'm sorry if my english is not very well..=)



my assignment is : Manufacture ofphosgenefromcarbonmonoxideandchlorine

I've discussed with my lecture at campus and I've discussed with Kostas in other thread too..
from them now my project look like this..

in reactor the reaction is :
CO _(gases) + Cl2 _(gases) --> COCl2 _(gases)

now, raw material CO stored in storage tank as gases phase,
and Cl2 stored as liquid phase then vaporized and heated upstream to reactor,
temperature of reaction 135 oC, and presseure 1 atm
the product COCl2 (phosgene) as gases too then condensed through a condensor so I got phosgene product as liquid.
from material balance I've got flowrate of product phosgene is 3156,5656 kg/hr, raw material CO 1300,908 kg/hr and Cl2 2206,3871 kg/hr


the problem now is :
CO stored as gases phase at temperature -185 oC, at atmospheric pressure
so, I must need a refrigerant to maintain it's temperature.
now i've decided to use liquid propane as refrigerant.
and the question now is, how much do I need refrigerant for CO storage tank..?
it's important for me to solve it's energy balance



as a comparison i'll attach the flowsheet
thx all for your help..  iso-8859-1__phosg.doc   25KB   6 downloads

[Absolute Zero]

I dont think that liquid propane can go upto -185C

[Bambang Wahyu Pramono]

propane melting point is -188 oC

[kkala]

Probably help to Banbang's topic needs some clarifications. It is a student project and the new query should have been posted to students' forum, but Banbang is apparently not familiar with that yet.
1. The original query is developed in the thread http://www.cheresour...-of-refrigerant, student forum. Storage condition of the two raw materials (CO and Cl2) was looked into. Instructor and group decided on atmospheric CO storage. This means liquid CO, not gas, so that storage volume of CO is reduced. Liquid CO has to be evaporated and superheated for phosgene synthesis.
2. The option of CO supply from pressure cylinders was apparently not preferred, due to their low capacity. Refrigerated storage for CO may or may not exist somewhere, it seems not economical in the specific case (low capacity), yet it looks technically possible since it is similar to a nitrogen atmospheric storage. Properties of CO versus N2 are respectively (values rounded to integers):
Normal boiling point : -191 versus -196 oC
Critical temperature: -140 oC versus -147 oC
Critical pressure : 35 bar a versus 34 bar a.
It is noted that this is a student project aiming at skill development, assumptions may not be so realistic.
3. I think Banbang should clarify approximate capacity of the storage, that is how many kg of N2 shall be stored in it (probably for a week production?). This could help for more specific answers.
4. Saturation temperature of propane even at 1 atm a is -42 oC (normal boiling point), so another refrigerant will be suitable. But I have no experience on refrigeration. Several possibilities / options of CO storage are touched in the first thread mentioned above, which can be reviewed for information.
5. Any advice on the topic would be helpful, improving basic knowledge.

Edited by kkala, 21 January 2012 - 12:48 PM.

[pavanayi]

Bambang,
Your choice of refrigerant is not proper. A refrigerant is preferred for a particular duty when its boiling point (not melting point) is less than the lowest process duty.
For example, if you wanted to cool a process down to -35°C, you would probably choose propane or propylene, which has a boiling point of around -40°C at atmospheric conditions, thereby having an approach temperature of 5°C. If you had a process that needs to be cooled to -90°C, you would choose a refrigerant like ethylene, which has a saturation temperature of about -100°C at atmospheric conditions.

Once you have selected the refrigerant based on temperature, you can calculate its mass flow rate. This will be based on how much cooling duty needs to be provided. In your case, it might be the amount of heat that leaks into the storage tank. The heat leak into the tank will lead to CO vaporising. This boil off gas will be condensed and put back into the tank. So the task for you will be to estimate the amount of liquid CO that is vaporising (and which will need to be condensed again) due to the heat leakage.

Remember: The refrigerant needs to be compressed and cooled, where it will condense. Not solidify. Then the liquid refrigerant is evaporated to achieve the cooling factor. If you are not comfortable with refrigeration, I would suggest you read a text book to read the fundamentals.

I understand from Kostas' post above that pressurised storage is not being considered.

For theoretical purpose, an option might be to use CO itself as the lowest refrigerant cycle to achieve a temperature of -190°C, with the storage being at slightly positive pressure to get enough approach temperature (For such applications, a 2°C approach might be reasonable). But then the problem lies in the fact that you will need to have further refrigeration cycles to reject the heat of the CO cycle into (typically termed as cascade refrigeration).
One link I found on internet for clarification purpose is
http://www.engr.sjsu...ion Systems.pdf
Check slides 2 and 3 in the above link for clarification of the idea.

For argument sake, considering pure refrigeraion, it might be like this: A CO refrigerant (cooling upto -190°C, the discharge pressure of CO compression being a bit less than 34 bar as per its critical properties). The CO refrigerant can be condensed using methane ( boiling point -161°C at atmospheric conditions). The methane cycle can be condensed at -82°C/45 bar according to its critical properties. The heat of methane cycle can be rejected into a ethane cycle, that into propane cycle and then to cooling water. The economics of the system might be very much skewed due to that particular approach.

Another point that you might want to consider is a mixed refrigerant cycle. (The refrigerant is a mixture of all the above pure refrigerants) and its composition adjusted for the requirement. Again, this topic might be a project in itself.

Edited by pavanayi, 21 January 2012 - 06:40 PM.

[kkala]

Post No 5 can be of real help, supplying detailed explanations on the topic. Indeed, the whole phosgene "project" has been rather complicated. I think there is even the simplifying option to flare boil off CO when phosgene reactor is out of operation, thus avoiding refrigeration, see http://www.cheresour...of-refrigerant/, posts No 11, 13.
Bambang (also knowing when the task should be accomplished) has to determine the scheme of CO storage, probably in communication with Instructor.

Edited by kkala, 22 January 2012 - 07:14 AM.